Technical note: On uncertainties in plant water isotopic composition following extraction by cryogenic vacuum distillation

Recent studies have challenged the interpretation of plant water isotopes obtained through cryogenic vacuum distillation (CVD) based on observations of a large H-2 fractionation. These studies have hypothesized the existence of an H-atom exchange between water and organic tissue during CVD extraction with the magnitude of H exchange related to relative water content of the sample; however, clear evidence is lacking. Here, we systematically tested the uncertainties in the isotopic composition of CVD-extracted water by conducting a series of incubation and rehydration experiments using isotopically depleted water, water at natural isotope abundance, woody materials with exchangeable H, and organic materials without exchangeable H (cellulose triacetate and caffeine). We show that the offsets between hydrogen and oxygen isotope ratios and expected reference values (delta H-2 and delta O-18) have inversely proportional relationships with the absolute amount of water being extracted, i.e. the lower the water amount, the higher the delta H-2 and delta O-18. However, neither delta H-2 nor delta O-18 values, were related to sample relative water content. The delta H-2 pattern was more pronounced for materials with exchangeable H atoms than with non-exchangeable H atoms. This is caused by the combined effect of H exchange during the incubation of materials in water and isotopic enrichments during evaporation and sublimation that depend on absolute water amount. The H exchange during CVD extraction itself was negligible. Despite these technical issues, we observed that the water amount-dependent patterns were much less pronounced for samples at natural isotope abundance and particularly low when sufficiently high amounts of water were extracted (> 600 mu L). Our study provides new insights into the mechanisms causing isotope fractionation during CVD extraction of water. The methodological uncertainties can be controlled if large samples of natural isotope abundance are used in ecohydrological studies.

Automated summary

Recent studies have questioned the interpretation of plant water isotopes obtained through cryogenic vacuum distillation (CVD) and proposed the hypothesis of H-atom exchange between water and organic tissue. We conducted experiments to test the uncertainties in the isotopic composition of CVD-extracted water and found that the offsets between hydrogen and oxygen isotope ratios were inversely proportional to the absolute amount of water being extracted, but not related to sample relative water content. The pattern of hydrogen isotope ratios was more pronounced for materials with exchangeable H atoms. However, if large samples of natural isotope abundance are used, the methodological uncertainties can be controlled.